1. Field of the Invention
The present invention relates to a display driving circuit and a display device using the circuit, and more particularly, to a driving circuit of a current-control light-emitting device such as an organic EL (electroluminescence) or LED (light emitting diode) in which an emitted-light brightness is controlled by a current circulating through an element, and a display device using the same.
2. Description of the Prior Art
Display devices are developed in each of which a matrix is formed by a scanning line and a data line and a light emitting element (pixel) such as an organic or inorganic EL is arranged to each intersection of the matrix to display image information. Because a display element itself emits light, these display devices do not require a backlight for illumination differently from a liquid-crystal display (LCD) and has a feature that there is no viewing angle dependency. Particularly, an active-driving display device in which an organic EL element and a transistor circuit are arranged has been particularly noticed in recent years and is expected as a display device substituted for an LCD because lower power consumption, high resolution, and high brightness can be obtained for a passive-driving display device constituted by only a light emitting element.
As a driving circuit of the active-driving display device, the following types are proposed: the voltage driving type for supplying a gradation voltage to each pixel, the current driving type for supplying a gradation current, and the digital driving type for controlling a light emitting period of a light emitting element. As a current-driving circuit, there is a circuit disclosed in “SID02 DIGEST (Euro Display 02), pp. 279˜282” (Document 1). It is possible to form the driving circuit by a low-temperature polysilicon (poly-Si) thin-film transistor and therefore, has a feature of forming a circuit on a glass substrate same as a display portion.
FIG. 13 is a block diagram of a display device including the driving circuit disclosed in the Document 1, in which a driving circuit for driving a display portion 103 is constituted by a reference-current circuit 101 and a current driver 102. The current driver 102 converts a digital image signal 52 input from an external unit as a digital value into a current. FIG. 5 shows a circuit block for constituting the current driver 102, in which the digital image signal 52 serially input from an external unit is sequentially captured by a logic circuit portion constituted by a shift register 3a, data register 3b, and a latching circuit 3c to explode digital image signals corresponding to the number of outputs of the current driver and then latches them. A D/I converter 3d converts the latched digital image signal into a current and supplies the current to the pixel circuit of the organic EL display portion 103 through a data line.
The D/I converter 3d has the configuration shown in FIG. 7 and is basically constituted by the current copier circuit 31 shown in FIG. 6. The D/I converter 3d is constituted by a plurality of one-bit D/I converting circuits. In the case of 6-bit gradation display, the D/I converter 3d is constituted by six one-bit D/I converting circuits. In FIG. 6, two current copier circuits are used every one bit because the current copier circuits execute two operations for storing a reference current and outputting the stored reference current.
That is, because the D/I converter 3d has a function for storing the reference current output from a reference current circuit 101 and outputting the stored reference current to a pixel circuit, it can be said that the converter 3d bears a part in acting as intermediary from the reference current adjusting circuit 101 to the pixel circuit. Therefore, the reference current serving as a current to be supplied to the pixel circuit is generated by the reference current circuit 101. The configuration in FIGS. 6 and 7 will be described later because it is the same as the case of an embodiment of the present invention.
FIG. 14 shows a general reference current circuit 101 for generating a 6-bit gradation current. The reference current circuit 101 is constituted by a current-setting transistor portion 2b constituted by N-channel transistors and a current mirror circuit portion 2c constituted by P-channel transistors. To set the ratio between reference currents i1 to i6 to 1:2:4:8:16:32, the channel widths of the N-channel transistors of the current setting transistors are set at the ratio of 1:2:4:8:16:32 or the N-channel transistors are connected in parallel at the ratio of 1:2:4:8:16:32.
However, a thin-film transistor for forming a driving circuit on a glass substrate has an astronomical variation between transistors compared to a transistor to be formed on a silicon substrate. Therefore, the accuracy of a reference current value output from the reference current circuit 101 is deteriorated due to the current variation in the current setting transistor portion 2b and the current variation in the current mirror circuit portion 2c and the ratio of 1:2:4:8:16:32 which is a designed value cannot be obtained. Therefore, there is a problem that a gradation display originally required cannot be performed.
To avoid the above problem, it is also considered to form the reference current circuit 101 on the silicon substrate by moving the circuit 101 to the outside of the glass substrate. However, sufficient gradation display cannot be obtained by this method because a reference current output line extends, thereby a parasitic capacitance applied to the line increases, and it is impossible to accurately supply the current at the low current side, that is, the lowbit side to the current driver 102. Moreover, as shown in FIG. 15, the accuracy of a reference current can be obtained by adjusting each gate voltage by an adjusting resistance portion 101c from the outside. In this case, however, the productivity is extremely deteriorated because special adjustment is required for each display device.
Referring to Japanese Patent Application Laid-Open No. 2001-324955, a voltage to be applied to a display element is controlled by comparing a current circulating through the display element with a reference current and conforming to the comparison result in order to restrain the variation of the brightness of the display element to a temperature change or a change with time. Moreover, in Japanese Patent Application Laid-Open No. 2002-229513, a brightness change is restrained by measuring the V/I (resistance) characteristic change of a display element due to a temperature change or change with time. Therefore, a small current is supplied to the display element to measure a voltage and correct and control the voltage applied to the display element.
However, also in the techniques disclosed in the Japanese Patent Application Laid-Open, it is clear that the accuracy of the above reference current value cannot sufficiently be obtained.